This invention relates to a gas sampling cell for use in the near simultaneous analysis of multiple gases by means of Raman scattering wherein the Raman scattering sample is contained in a cell placed within the laser resonator. More specifically, this invention relates to a gas cell unit for use in the detection of multiple respiratory and anesthesia gases by Raman scattering wherein the incident laser beam passes through the gas sample contained in the cell which is preferably placed in the intracavity of a laser. However, the cell could also be used for extra cavity laser Raman gas determinations.
A proposed method for use in monitoring several gases in critical care situations is based on Raman light scattering. The Raman light scattering effect relies on the interaction of monochromatic light with the vibrational/rotational modes of molecules to produce scattered light which is frequency shifted from that of the incident radiation by an amount corresponding to the vibrational/rotational energies of the scattering molecules. Since these energies are species-specific, an analysis of the various frequency components present in the Raman scattering spectrum provides chemical identification of the gases present in the scattering volume. The intensity of the various frequency components or Raman lines provides quantitation of the gases present providing suitable calibrations have been made. The relative sensitivity to the different gases remains absolutely fixed, eliminating frequent calibration requirements.
Raman techniques have been widely used for atmospheric monitoring and for combustion applications. Sensitivities better then 1 ppm have been demonstrated. Typical application of Raman scattering analysis coupled with computer assisted signal processing techniques is reported in Lapp et al., "Laser Raman Gas Diagnostics", Plenum Press, New York. London, 1974.
Raman scattering analytical techniques are also described in the patent literature. Chupp, U.S. Pat. No. 3,704,951 teaches laser Raman spectroscopy utilizing a sampling cell with a multi-pass configuration. A laser beam enters into the cell configurations of concave mirrors facing each other such that there is a multiple reflection of the laser beam between the mirrors to accomplish the required optical power density enhancement in the sampling area and subsequent signal enhancement. This device and accompanying technique is limited in that it provides for analysis through only a single detector. Hence, simultaneous monitoring of multiple gases is not possible. Moreover, this device is intended for use primarily with liquids and has only limited application for gases. Also, the alignment of the mirrors for optimal signal is exceedingly delicate. Finally, the beam size in the sampling region must be quite small to maintain low sample volume and subsequently high signal response time. A multimirror approach makes this difficult, if not impossible, given the optics of such a system.
Hatzenbuhler, U.S. Pat. No. 3,807,862 also teaches a specific application of Raman spectroscopy in which a fluid sample is subjected to a laser beam and only a single Raman line is evaluated. In other words, there is no teaching of a technique for the determination of multiple gases.
Leonard, U.S. Pat. No. 3,723,007 is drawn to a method for the remote sensing of gas concentrations through use of a high-energy pulsed laser and a mirror telescope, using a grid polychromator. This system requires a laser output in the 10 kW range and is unsuitable for general application. Moreover, the use of an expensive spectrometer presents an obstacle in the way of cost-beneficial production of the device.
A more recent and effective system for the simultaneous detection of multiple gases is taught in Albrecht, et al., German Pat. DE No. 27 23 939 C2. This patent also utilizes a multi-pass cell to constrain the laser radiation in a region between two concave mirrors for signal enchancement but utilizes an unpolarized laser beam to provide a 360.degree. monitoring geometry for the Raman scattered light. A series of six detectors, each accompanied by an interference filter comprised of one broad-band and one gas-specific filter, are provided to collect six separate Raman lines for the simultaneous monitoring of six different gas components. This method, while monitoring multiple gases simultaneously, requires six separate detectors including separate photomultiplier tubes and recording instruments. Such a complex system is bulky and expensive. Moreover, since the orientation of the six detectors described in the German patent could not be expected to exactly image in the same area, the acquisition of all gas concentrations could not be from exactly the same point in the gas flow stream.
A method and system for the near simultaneous monitoring and analysis of multiple gases which avoids the use of multiple detectors is disclosed in an application entitled "Molecular Gas Analysis By Raman Scattering In Intracavity Laser Configuration" filed Sept. 11, 1985 as Ser. No. 774,643. The present inventor is one of the joint inventors of that application which discloses a gas cell and outer casing for use in the intracavity laser sampling and analysis technique. That cell and casing is the sole invention of this inventor and forms part of the subject matter of this application.